Intermittent strong vibration

Purchased a 1997 Ford Explorer XLT, V6 new in '97. Has been very reliable and serviced at the dealer since purchased. Has 114,000 miles. Has recently develop a very strong vibration at 55 mph and faster. It lasts for 2 - 3 miles and stops. It is not present all the time. Vibration is so strong, it’s like running on rumble strips. Shakes the contents on the counsel. Alignment is OK, tires are worn evenly, and check engine light has never come on. Technicians at Ford service cannot find problem. Did not vibrate during their test drive. Last time I drove the car, the vibration occurred twice in a 15 mile drive.